Confused With Planting Substrate.

Compost must be capped off with a layer of substrate, otherwise the nitrogen will leech into the water column.

The rest is down to opinion and personal preference.

I assume you've already considered and dismissed the special substrates designed for planted aquariums, such as those from JBL and Tropica etc. (they are quite expensive!)

In my opinion sand isn't ideal because it doesn't provide enough for plant's roots to anchor into.

I used fine natural gravel and like the look and the plants are doing well.

I don't have compost and have been kicking myself since learning of this method.

I use root tabs to enrich the gravel but if I started again I'd use about 20mm of soil with about 30mm of gravel.

There's an article here which you might find useful.
http://www.ukaps.org/forum/threads/the-soil-substrate-or-dirted-planted-tank-a-how-to-guide.18943/

I've posted the part about soil depth and capping substrate depth below:

Soil depth
Well that’s the basic components explained all we need to do now is put them together. Start by placing your slightly moistened soil in the bottom of the tank to a depth of around 2cm, you can slope it up toward the back to 4cm or so if desired.
...

Capping substrate
Cap the whole lot with about 3 cm of well washed gravel or sand, sloping to perhaps 4cm – 5cm at the back.
...

No problem Alex. 

I hope you're planning on posting photos when you've finished.

Hi Alex

I have compost capped off with sand.  My planted tank has been up and running for 13months now without problems other than I got a bit carried away with the number of plants I put in and had to thin them out over time.  The Fish were running out of room to swim 

The reason I choose sand was that I wanted peppered cories and it's ideal for them to dig in with out damaging their barbels/whiskers.  As they dig and sift through the sand it trickles out their gills!  I also have MTS to keep the sand turned over so that it doesn't get compacted/pockets of gas forming.  Another way to prevent the sand/ compost from becoming compacted is to put large rocks in first and build the compost/sand around them.

Anne

Thanks for your input Anne and Richard.

I did want cories in mine as well.

I did find a gravel that was very fine at p@h very similar to a sand grain but about 2/3 mm in size. Which I thought was perfect, until I read the reviews, this particular gravel leached it's colouring and clouded the customers tanks. http://www.petsathome.com/shop/en/pets/black-aquarium-gravel

Thanks Richard, I'll have a look for your recommendation of gravel.